spl_mmc: add Falcon mode support for FAT variant

If Falcon mode support is enabled (and the system isn't directed into
booting u-boot), it will instead try to load kernel from
CONFIG_SPL_FAT_LOAD_KERNEL_NAME file and kernel argument parameters from
CONFIG_SPL_FAT_LOAD_ARGS_NAME, both from the same partition as u-boot.

Signed-off-by: Peter Korsgaard <peter.korsgaard@barco.com>